This invention relates to electricity meters such as used by commercial, industrial, or residential customers of power utility companies and, more particularly, to a revenue accuracy meter having various operational capabilities such as power quality measurement and/or energy management.
Utility power distribution generally starts with generation of the power by a power generation facility, i.e., power generator or power plant. The power generator supplies power through step-up subtransmission transformers to transmission lines. To reduce power transportation losses, the step-up transformers increase the voltage and reduce the current. The actual transmission line voltage conventionally depends on the distance between the subtransmission transformers and the users or customers. Distribution substation transformers reduce the voltage from transmission line level generally to a range of about 2-35 kilo-volts (xe2x80x9ckVxe2x80x9d). The primary power distribution system delivers power to distribution transformers that reduce the voltage still further, i.e., about 120 V to 600 V.
For background purposes, and future reference herein, an example of a power utility distribution system as described above and understood by those skilled in the art is illustrated in FIGS. 1A and 1B of the drawings. Power utility companies, and suppliers thereto, have developed systems to analyze and manage power generated and power to be delivered to the transmission lines in the primary power distribution system, e.g., primarily through supervisory control and data acquisition (xe2x80x9cSCADAxe2x80x9d). These primary power distribution analyzing systems, however, are complex, expensive, and fail to adequately analyze power that is delivered to the industrial, commercial, or residential customer sites through the secondary power distribution system.
Also, various systems and methods of metering power which are known to those skilled in the art are used by commercial, industrial, and residential customers of power utility companies. These power metering systems, however, generally only measure the amount of power used by the customer and record the usage for reading at a later time by the utility power company supplying the power to the customer. A revenue accuracy meter is an example of such a metering system conventionally positioned at a customer site to receive and measure the amount of power consumed by the customer during predetermined time periods during a day.
Conventionally, electric power is delivered to industrial, commercial, and residential customers by local or regional utility companies through the secondary power distribution system to revenue accuracy type electricity meters as an alternating current (xe2x80x9cACxe2x80x9d) voltage that approximates a sine wave over a time period and normally flows through customer premises as an AC current that also approximates a sine wave over a time period. The term xe2x80x9calternating waveformxe2x80x9d generally describes any symmetrical waveform, including square, sawtooth, triangular, and sinusoidal waves, whose polarity varies regularly with time. The term xe2x80x9cACxe2x80x9d (i.e., alternating current), however, almost always means that the current is produced from the application of a sinusoidal voltage, i.e., AC voltage.
In an AC power distribution system, the expected frequency of voltage or current, e.g., 50 Hertz (xe2x80x9cHzxe2x80x9d), 60 Hz, or 400 Hz, is conventionally referred to as the xe2x80x9cfundamentalxe2x80x9d frequency, regardless of the actual spectral amplitude peak. Integer multiples of this fundamental frequency are usually referred to as harmonic frequencies, and spectral amplitude peaks at frequencies below the fundamental are often referred to as xe2x80x9csub-harmonics,xe2x80x9d regardless of their ratio relationship to the fundamental.
Various distribution system and environmental factors, however, can distort the voltage waveform of the fundamental frequency, i.e., harmonic distortion, and can further cause spikes, surges, or sags, and other disturbances such as transients, time voltage variations, voltage imbalances, voltage fluctuations and power frequency variations. Such events are often referred to in the art and will be referred to herein as power quality disturbances, or simply disturbances. Power quality disturbances can greatly affect the quality of power received by the power customer at its facility or residence.
These revenue accuracy metering systems have been developed to provide improved techniques for accurately measuring the amount of power used by the customer so that the customer is charged an appropriate amount and so that the utility company receives appropriate compensation for the power delivered and used by the customer. Examples of such metering systems may be seen in U.S. Pat. No. 5,300,924 by McEachern et al. titled xe2x80x9cHarmonic Measuring Instrument For AC Power Systems With A Time-Based Threshold Meansxe2x80x9d and U.S. Pat. No. 5,307,009 by McEachern et al. titled xe2x80x9cHarmonic-Adjusted Watt-Hour Meter.xe2x80x9d
These conventional revenue accuracy type metering systems, however, have failed to provide information about the quality of the power received by a power customer at a particular customer site. Power quality information may include the frequency and duration of power quality disturbances in the power delivered to the customer site. As utility companies become more and more deregulated, these companies will likely be competing more aggressively for power customers, particularly heavy power users, and therefore information regarding the quality of the power received by the power customer is likely to be important.
For example, one competitive advantage that some utility companies may have over their competitors could be that their customers experience relatively few power quality disturbances. Similarly, one company may promote the fact that it has fewer times during a month that power surges reach the customer causing potential computer systems outages at the customer site. Another company may promote that it has fewer times during a month when the voltage level delivered to the customer is not within predetermined ranges which may be detrimental to electromagnetic devices such as motors or relays. Previous systems for measuring quality of power in general, however, are expensive, are bulky, require special set up and are not integrated into or with a revenue accuracy meter. Without a revenue accuracy metering system that measures the quality of the power supplied to and received by the customer, however, comparisons of the quality of power provided by different suppliers cannot readily be made.
One solution to the above described problems is proposed by U.S. Pat. No. 5,627,759 to Bearden et al. (hereinafter the xe2x80x9cBearden patentxe2x80x9d), which is assigned to the assignee of the present invention and incorporated herein by reference. The Bearden patent describes a revenue accurate meter that is also operable to, among other things, detect power quality events, such as a power surge or sag, and then report the detection of the power quality event to a utility or supplier.
One of the useful features of the meter disclosed in the Bearden patent is waveform capture. The meter of the Bearden patent is operable to obtain waveform information regarding the voltage and/or current waveform at about the time a power quality event is detected. Such a feature is advantageous because the captured waveform may be analyzed to help determine potential causes of the event, the severity of the event, or other pertinent data. While the waveform capture feature disclosed by the Bearden patent contributes to the usefulness of the meter, the increased sophistication of power consumers has created a need for further information retrieval capabilities in power quality measurement devices.
In particular, it has been found that much may be learned about a power quality event by analyzing the voltage waveform when the power quality event ends, as well as when it begins. Moreover, it has been found that power quality events can often include one or more xe2x80x9csub-eventsxe2x80x9d. For example, consider a power quality event in the form of a voltage sag wherein the line voltage falls from 120 volts to 100 volts for five minutes, and then falls to 85 volts for two hours, and then returns to 100 volts for an hour, and then returns to 120 volts. It is useful to gather information about such sub-events for analysis of the power distribution system.
One solution would be to implement a power quality device that captures all the voltage waveform data at all times. However, capturing all such data is impracticable. In particular, in order to be of use, the voltage waveform data must be captured, or in other words, stored in non-volatile memory for subsequent retrieval and analysis. The totality of waveform data for any significant length of time over one second is substantial. For example, at a sampling rate of 32 samples per cycle, one seconds worth of data for a single voltage waveform constitutes 1920 samples of data per second. For poly-phase meters wherein both voltage and current are sampled, the number of samples is four to six times that number.
It is thus apparent that the storage of ongoing waveform data cannot be achieved in non-volatile memory contained within the meter for any significant period of time. Moreover, it is impracticable for the meter to communicate such data to a remote device having greater memory capability. In particular, the communication of such amounts of data requires a substantial amount of constantly available communication bandwidth, which is not cost-effective nor practical.
What is needed, therefore, is a power measurement device which is coupled with a revenue accurate meter and has the capability to obtain waveform information for multiple phenomena within a power quality event. In particular, there is a need for such a device that obtains waveform information for both the beginning and the end of a power quality event, as well as for a device that obtains waveform information for a plurality of sub-events within a particular power quality event.
The present invention addresses the above needs, as well as others, by providing a system and method for use in a revenue accurate meter that detects a variation in the line voltage level from a normal voltage level, capturing the line voltage waveform corresponding to the time when the variation was detected, detecting a reduction in the variation of the line voltage level from the normal voltage level, and capturing the waveform corresponding to the time the reduction of the variation was detected. By capturing waveforms both at the time that the voltage varies from the norm and the time that the voltage returns to the norm, the present invention obtains further detailed information about a power quality event than that available in the prior art. Moreover, by capturing waveforms corresponding to the times that the voltage variation is detected and the time that the reduction in the variation is detected, storage and/or communication of the captured waveform data is practicable. Such data may then be analyzed at a later time and/or at a remote location to obtain information regarding the disturbance that caused the line voltage variation.
An exemplary method according to the present invention is carried out within an electrical energy meter containing means therein for metering a quantity of electrical energy generated by a supplier and transferred via a power supply line to a load of a customer during an energy measurement time interval. The exemplary method is a method of monitoring variations in the metered quantity of electrical energy.
The method includes a first step of sensing a line voltage transferred via the power supply line to the load during the energy measurement time interval. The method also includes the step of detecting a variation in a magnitude of the sensed line voltage relative to an acceptable voltage level, wherein said variation exceeds a first variation threshold. The method then includes the step of capturing a first waveform of the sensed line voltage corresponding to the time when said variation is detected. Finally, the method includes the steps of detecting a subsequent reduction in the variation of the magnitude of the sensed line voltage such that the variation is equal to or less than the first variation threshold and capturing a second waveform of the sensed line voltage corresponding to the time when the subsequent reduction in the variation is detected.
An exemplary apparatus according to the present invention is an electrical energy meter for obtaining data regarding line voltage variations in real-time. The electrical energy meter includes a voltage digitizing circuit, a current digitizing circuit, a metering circuit and a power quality circuit. The voltage digitizing circuit is operable to obtain analog line voltage information and generated digital line voltage information therefrom. The current digitizing circuit is operable to obtain analog line current information and generate digital line current information therefrom. The metering circuit is operable to receive the digital line voltage information and the digital line current information and generate metering information therefrom.
The power quality circuit is operable to: receive the digital line voltage information and obtain magnitude information therefrom, the magnitude information representative of the magnitude of the line voltage; detect a variation in the magnitude of the line voltage relative to an acceptable voltage level wherein said variation exceeds a first variation threshold; capture a first waveform in the form of a first set of digital line voltage information corresponding to the time when said variation is detected; detect a subsequent reduction in the variation of the magnitude of the line voltage such that the variation is equal to or less than the first variation threshold; and capture a second waveform in the form of a second set of digital line voltage information corresponding to the time when the subsequent reduction in the variation is detected.
The exemplary method and apparatus described above provide the above mentioned advantages of capturing waveform information for a voltage waveform at both the beginning of a power quality event and the end of a power quality event. In alternative embodiments, the method and apparatus may be configured to detect a second variation that is greater than the first variation, and capture a waveform corresponding to the time when the second variation is detected. Such a method and apparatus would then be capable of capturing waveform data on sub-events within a power quality event.
The above features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.